An inverter unit is a power conversion device based on power electronic technology for converting electric energy from DC to AC or from AC to DC. Typically, modular parallel power system architectures have been widely applied, in order to facilitate expansion and make redundant designs or reach other purposes. As shown in FIG. 1, it is a schematic diagram of the structure of an inverter system in the related art. The inverter system in the prior art comprises N (N≧1) inverter units (VSCs), which are connected in parallel for supplying power for the local load.
For such a parallel power system, it is necessary to control evenly distribution of the load power among the N inverter units, as well as make the load voltage Vmg (i.e. a feedback signal Vmg reflecting load voltage) stabilize at a certain reference value Vref, for example, the reference value can be a constant sine wave at 220V and 50 Hz. One control objective is called load power sharing, and the other control objective is called voltage regulation.
Regarding the control objective of the voltage regulation, a voltage closed-loop control method is generally adopted, for example, multiple closed-loop control method using voltage in the outer loop and current in the inner loop, etc. As long as the band width of the voltage loop is designed to be wide enough, voltage control accuracy can be guaranteed. This is easy for a single inverter unit; however, when a plurality of inverter units are connected in parallel, the load voltage is co-determined by all the inverter units. Moreover, the output power of each of the inverter units is co-determined by its output voltage and output impedance. Since the respective component parameters and other parameters, such as line impedance, of the inverter units are very hard to be exactly the same in practice, the inverter units need to output different voltages in order to achieve power sharing. Therefore, it is not easy to fully achieve voltage regulation and power sharing objectives by simply setting the same voltage command for each inverter unit for voltage closed-loop control.
In the related art, droop control is one way to achieve the power sharing of parallel inverter units. The so-called droop control means that the output voltage command of the inverter unit varies as the output power changes, and typically manifests as a drooping curve. FIG. 2(a) is a schematic diagram of an active power-output voltage frequency curve. For example, in the case of inductive output impedance, the frequency ω of the output voltage of an inverter unit decreases as the active power P outputted by the inverter unit increases. FIG. 2(b) is a schematic diagram of a reactive power-output voltage amplitude curve, wherein the amplitude V of the output voltage of an inverter unit decreases as the reactive power Q outputted by the inverter unit increases. Moreover, in order to achieve a better power sharing effect, a drooping curve will have a greater tilt at the expense of voltage regulation factor, i.e., load will have a greater impact on the output voltage. That is to say, voltage regulation and power sharing cannot be achieved simultaneously, and a compromise often needs to be made for the design of an inverter system.
Thus it is necessary to find a solution on achieving effective power sharing and good voltage regulation simultaneously.